1. Field of the Invention
This invention relates to high pressure metering pumps for primary use in laboratory analysis routines.
2. Description of Related Art
In laboratory analysis equipment such as high pressure liquid chromatography (HPLC), a fundamental requirement for successful analysis is a metering pump that can deliver extremely high pressure (several thousand psi) and at a very small (microliters/minute), reliably constant flow rate. Of course, the pumping equipment must be completely free of any potential for contaminating the solvent used in the HPLC run. This solvent may often comprise a corrosive liquid; as a result, the materials that form the pump surfaces contacting the solvent must be chosen with great care.
Typically, high pressure metering pumps use a movable piston and dynamic seal to provide positive displacement flow. The disadvantage of this arrangement is that the seals generate wear material, especially as the piston wears out, an especially severe problem when running buffers, or salt solutions (very common). This wear material not only damages downstream hydraulic components, but also clogs the inlet and outlet check valves. Thus the number one and two problems in HPLC instrumentation are valve failure and seal failure.
One state of the art metering pump uses a stainless steel diaphragm that acts as sealing barrier, with pressurized oil on the pumping side of the diaphragm to support the high pressures. A notable failure point of this type of construction is the diaphragm, due to the finite fatigue life of a thin metal member that is experiencing continuous flexure under high stress, and eventually it will fail and allow the backup oil to leak into the chromatography solvent stream and destroy sensitive hydraulic components such as the column. Another disadvantage is the added complexity of this arrangement: it requires another pumping piston and seal to pressurize/depressurize the oil behind the diaphragm. In some other prior art metering pumps, there is no backup provision of any kind to support the diaphragm, causing the diaphragm to experience the maximum pressure differential across its thin, flexible, widespread extent. This factor inherently limits the pump output pressure in such devices to less than a few hundred psi.